John Maynard Keynes famously warned that "the markets can remain irrational longer than you can remain solvent." Considering the recent volatility in stocks worldwide, irrationality appears to be the order of the day. Rarely have investors been so prone to bouts of panic selling, punctuated by spasms of equally frenzied buying. The Dow Jones Industrial Average index lost nearly a quarter of its value between Oct. 1 and Oct. 27, including an epic 700-point drop on Oct. 15. The carnage was followed on Oct. 28 by a 900-point rise, the second largest points gain on...